In general, a battery is a device that converts the chemical energy contained in its active materials directly into electric energy by means of an electrochemical oxidation-reduction (redox) reaction. This type of reaction involves the transfer of electrons from one material to another through an electric circuit.
While the term “battery” is often used, the basic electrochemical unit being referred to is the “cell” (not to be confused with a structural cell, to be discussed below). A battery comprises one or more of these battery cells, connected in series or parallel, or both, depending on the desired output voltage and capacity.
The battery cell comprises three major components: anode or negative electrode; cathode or positive electrode; and the electrolyte. The anode—the reducing or fuel electrode—gives up electrons to the external circuit and is oxidized during the electrochemical reaction. The cathode or positive electrode—the oxidizing electrode—accepts electrons from the external circuit and is reduced during the electrochemical reaction. The electrolyte—the ionic conductor—provides the medium for transfer of electrons as ions, inside the cell between the anode and cathode. The electrolyte is typically a liquid, such as water or other solvent, with dissolved salts, acids, or alkalis to impart ionic conductivity. Some batteries use solid electrolytes, which are ionic conductors at the operating temperature of a cell (See D. Linden, “Handbook of Batteries,” 2nd edition, McGraw-Hill, Inc., New York, (1995), of which is hereby incorporated by reference herein). Other batteries are disclosed in U.S. Pat. Nos. 6,010,543 and U.S. Pat. No. 5,916,706 to Berkey et al. and No. 4,160,068 to Kummer; all of which are incorporated by reference herein.
Physically the anode and cathode electrodes are electronically isolated in the cell to prevent internal short-circuiting, but are surrounded by the electrolyte. In practical cell designs a separator material is used to separate the anode and cathode electrodes mechanically. The separator, however, is permeable to the electrolyte in order to maintain design. Electrically conducting grid structures or materials may also be added to the electrodes to reduce internal resistance.
The battery cell configurations typically have included—cylindrical, button, flat, and prismatic—and the cell components are designed to accommodate the particular cell shape. The cells are sealed in a variety of ways to prevent leakage and dry-out. Some cells are provided with venting devices or other means to allow accumulated gases to escape. Suitable cell cases or containers and means for terminal connection are added to complete the cell.
FIG. 1A schematically shows the operation of a battery cell 1 during discharge. When the cell 1 is connected to an external load, electrons flow from the anode 6, which is oxidized, thought the external load to the cathode 8, where the electrons are accepted and the cathode material is reduced. The electric circuit 9 is completed in the electrolyte 12 by the flow of anions (negative ions) and cations (positive ion) to the anode 6 and cathode 8, respectively. The battery may include a casing 2, which can be hermetically sealed and/or can include a vent 4. A separator 10 separates the anode 6 and cathode 8. The electrolyte may be an aqueous solution consisting of mainly potassium hydroxide (KOH).
As shown in FIG. 1B, during the recharge of a rechargeable or storage battery 1, the current flow is reversed and oxidation takes place at the positive electrode and reduction at the negative electrode. As the anode is, by definition, the electrode at which oxidation occurs and the cathode the one where reduction takes place, the positive electrode is now the anode and the negative is the cathode during recharge.
With regards to the present invention, the rechargeable sealed nickel-metal hydride battery is a relatively new technology with characteristics similar to those of the sealed nickel-cadmium battery. The principal difference is that the nickel-metal hydride battery uses hydrogen, absorbed in a metal alloy, for the active negative material in place of the cadmium used in the nickel-cadmium battery.
The metal hydride electrode has a higher energy density than the cadmium electrode. Therefore, the amount of the negative electrode used in the nickel-metal hydride cell can be less than that used in the nickel-cadmium cell. This allows for a larger volume for the positive electrode, which results in a higher capacity or longer service life for the metal hydride battery. Furthermore, as the nickel-metal hydride battery is free of cadmium, it is considered more environmentally friendly than the nickel-cadmium battery and may reduce the problems associated with the disposal of rechargeable nickel batteries.
The sealed nickel-metal hydride battery is often preferred compared with the sealed nickel cadmium cell for use in computers, cellular phones, and other portable and consumer electronics applications where the higher specific energy is desired. The metal hydride battery in larger sizes is also utilized for use in applications such as electric vehicles, where its higher specific energy and good cycle life approach critical performance requirements.
The active metal of the positive electrode (cathode) of the nickel-metal hydride battery, in the charged state, is nickel oxyhydroxide (NiOOH). This is the same as the positive electrode in the nickel-cadmium battery.
The negative (anode) active material, in the charged state is hydrogen in the form of a metal hydride W). This metal alloy is capable of undergoing a reversible hydrogen absorbing-desorbing reaction as the battery is charged and discharged.
An aqueous solution of potassium hydroxide KOH) is the major component of the electrolyte. A minimum amount of electrolyte is used in this sealed cell design, with most of the liquid absorbed by the separator and the electrodes. This “starved-electrolyte” design, similar to the one in sealed nickel-cadmium batteries, facilitates the diffusion of oxygen to the negative electrode at the end of the charge for the oxygen-recombination reaction. This is essentially a dry-cell construction, and the cell is capable of operating in any position.
During discharge, the nickel oxyhydroxide (NiOOH) is reduced to nickel hydroxide (Ni(OH)2),NiOOH+H2O+e→Ni(OH)2+OH−and the metal hydride (MH) is oxidized to the metal alloy M,MH+OH−→M+H2O+eThe overall reaction on discharge isMH+NiOOH→M+Ni(OH)2 The process is reversed during charge.
Two types of metallic alloys are generally used for the negative electrode (anode). These are the rare-earth (Misch metal) alloys based around lanthanum nickel, known as the AB5 class of alloys, and alloys consisting of titanium and zirconium, known as the AB2 class of alloys. In both cases, some of the base metals are replaced with other metals to achieve the desired characteristics.
The AB5 alloys are essentially based on LaNi5, with various substituents for lanthanum and nickel to stabilize the alloy during charge/discharge cycling by reducing the internal absorption and/or forming protective surface films. For example, the volume expansion is reduced by a partial substitution of Ni with Co and the interfacial properties improved with small amounts of Al or Si. The cycle life improves upon the substitution of Ni with the ternary solute in the order Mn<Ni<Cu<Cr<Al<Co. A substitution of the rare-earth metal site with Ti, Zr, or other lanthanides such as Nd and Ce render the formation of a protective surface film and enhance the cycle life. This eventually led to the use of relatively inexpensive mish metal, Mm, a naturally occurring mixture of rare-earth metals (mainly La, Ce, Pr, and Nd) in place of La. The use of misch metal also improved the durability of the alloy, as evident from the long cycle life as well as the quantitative estimates of the surface layers [La(OH)3 and Mm(OH)3] on the cycled electrodes.
The AB2 alloys have been improved by using vanadium-titanium-zirconium-nickel based alloys.
Sealed nickel-metal hydride cells are constructed in cylindrical, button, and prismatic configurations, similar to those used for the sealed nickel-cadmium battery.
With regards to construction, the electrodes are designed with highly porous structures having a large surface area to provide a low internal resistance and a capability for high-rate performance. The positive electrode in the cylindrical nickel-metal hydride cell is a highly porous sintered, or felt nickel substrate into which the nickel compounds are impregnated or pasted and converted into active material by electrodeposition. The negative electrode, similarly, is a highly porous structure using a perforated nickel foil or grid onto which the plastic-bonded active hydrogen storage alloy is coated. The electrodes are separated with a synthetic nonwoven material, which serves as an insulator between the two electrodes and as a medium for absorbing the electrolyte.
With regards to the electrical aspects, the NiMH battery produces a nominal voltage of about 1.2–1.3 volts. The total voltage of the battery system can be provided in multiples of this voltage by providing battery cells in series with each other. For example, placing ten such battery cells in series would provide a battery voltage of about 12 to 13 volts. The voltage of the electrochemical reaction is affected by ambient temperature. The effect on voltage is a function of the temperature in degrees Kelvin to a multiple power. Thus, across typical atmospheric temperature variations, the voltage does not vary radically. Further, as the temperature becomes too high the increased benefits from increased voltage from the reaction are offset by the problem of excessive chemical corrosion of the battery components.
The total battery capacity or energy density is dependent on the amount of active metal in the cathodes and anodes. In other words, how many active metal molecules are available to give up an electron.
For redundancy purposes and in order to prevent failed battery cells which are connected in parallel to other battery cells from adversely affecting those battery cells, diodes, circuit breakers and relays can be used to control and prevent short circuiting in one circuit from adversely effecting an adjacent parallel circuit.
The total surface area or amount of reagent determines the current produced by the battery system in the battery system. Thus, for a given battery cell size, the current produced by the battery system can be increased by placing additional battery cells in parallel with each other. The current is also a function of the spacing between the anode and cathode. The current within a given battery cell is governed by the availability of ions from the electrolyte at the surfaces of the electrodes. This, in turn is governed by the distance ions must drift through the electrolyte to cross the separator. With this design, the spacing is smaller, due to the smaller effects of material expansion as cited earlier. Higher charge and discharge currents may be achieved.
U.S. Pat. No. 5,567,544 to Lyman discloses a battery having an anode, a cathode, and a separator formed into a honeycomb structure; of which is hereby incorporated by reference in its entirety. While the Lyman battery & structure is compatible to fit into various linear like frames it fails to provide the structural integrity, strength, design flexibility, and fabrication simplification of the present invention battery, or related method of making the same.
There is therefore a need in the art for an effective battery, and method of producing the same, having multifunctional structures that combine significant load bearing support in addition to electrochemical energy storage.